Wall thinning due to corrosion in utility boiler water-wall tubing is a significant operational concern for boiler operators. Historically, conventional ultrasonics have been used for inspection of these tubes. However, ultrasonic inspection is very manpower intensive and a slow process. Therefore, thickness measurements are typically taken over a relatively small percentage of the total boiler wall and statistical analysis is used to determine the overall condition of the boiler tubing. Other inspection techniques such as electromagnetic acoustic transducer (EMAT), have been evaluated, however they provide only a qualitative evaluation by identifying areas or spots where corrosion has significantly reduced the wall thickness.
In U.S. Pat. No. 6,000,844, which is incorporated herein in its entirety by reference, a method and a portable apparatus are disclosed for the non-destructive identification of defects in structures. The apparatus includes a heat source and a thermal imager that move at a constant speed past a test surface of a structure. The thermal imager is offset at a predetermined distance from the heat source. The heat source induces a constant surface temperature, preferably along a continuously advancing line or narrow region on the material being tested. The imager follows the heat source and produces a video image of the thermal characteristics of the test surface. Material defects produce deviations from the constant surface temperature that move at the inverse of the constant speed. Thermal noise produces deviations that move at random speed. Computer averaging of the digitized thermal image data with respect to the constant speed minimizes noise and improves the signal of valid defects. The motion of thermographic equipment coupled with a high signal-to-noise ratio renders it suitable for portable, on-site analysis.